1. Field of the Invention
This invention relates to an improved process for the recovery of rhenium values from aqueous molybdenum solutions containing rhenium. In particular, it relates to a process for the recovery of rhenium from sodium molybdate solutions prepared from molybdenite concentrates.
2. Prior Art
Rhenium is often present in small amounts in molybdenite concentrates. For example, molybdenite concentrates formed as a by-product from the recovery of other metals from ores such as copper contain small amounts of rhenium, generally in order of from 0.4 to 4 pounds per ton of concentrate. Recovery of even these rhenium values can aid appreciably in lower overall manufacturing costs because of the high cost of rhenium. Usually these concentrates are roasted to recover the molybdenum as molybdenum oxide and the rhenium values are volatilized. The rhenium values are then recovered from the flue dusts and gases by contacting them with water or some other solvent, such as an acidic or basic medium. The rhenium values are then extracted from such rhenium bearing solutions by ion-exchange. According to U.S. Pat. No. 2,876,065, the impure clarified rhenium solution is passed through ion-exchange equipment in intimate contact with a strongly basic alkylamine-type synthetic resin. Examples of commercially available resins are Amberlite IRA-400 and Dowex 1.times.8. After elution to remove molybdenum and other impurities, the rhenium is eluted with a strong mineral acid such as perchloric acid. Rhenium sulfide (Re.sub. 2 S.sub.7) is precipitated from the rhenium-bearing acid solution by H.sub.2 S gas. The rhenium sulfide is then oxidized and converted to NH.sub.4 ReO.sub.4 by the addition of NH.sub.4 OH. The NH.sub.4 ReO.sub.4 is crystallized by evaporation.
A similar process is disclosed in U.S. Pat. No. 3,244,475 except that a liquid ion-exchange material is used instead of an ion exchange resin to recover the rhenium. The rhenium values are recovered from the aqueous solution by means of a quaternary ammonium compound carried in kerosene with primary decyl alcohol as the solubilizer. The foregoing process is also described in the Bureau of Mines Report of Investigation #6246 entitled "Sources and Recovery Methods for Rhenium" by the inventors of the foregoing patent. The amine is stripped with dilute perchloric acid and NH.sub.4 ReO.sub.4 is recovered in much the same manner as is disclosed in U.S. Pat. No. 2,876,065. In the foregoing report #6246, it is disclosed that various acids and salt solutions were tried and only perchloric acid and perchlorate salt solutions were effective in removing rhenium from the organic. Under the conditions disclosed in the patent, the extraction efficiency was lowered upon recycle of the extraction system. For example, as shown in Example 1 in the original pass 142 mg. of rhenium was extracted out of 152 mg., or 94.5 percent of the rhenium, and upon recycle, the recycle extracted only 85 percent of the rhenium. The foregoing Bureau of Mines Report also states that reuse of the organic results in lower extraction efficiencies particularly when perchloric acid was used as the stripping agent.
More recently, however, the U.S. Pat. No. 3,558,268 discloses a process similar to the above processes in which ammonium thiocyanate (NH.sub.4 SCN) is used as the stripping agent and crystallization of NH.sub.4 ReO.sub.4 directly from the NH.sub.4 SCN solution is claimed, thereby eliminating the sulfide precipitation and oxidation steps.
There are several disadvantages with the foregoing processes which result in recovery appreciably less than all of the rhenium initially present in the solutions. For example, the molybdenum must be separated from the rhenium by roasting prior to the recovery of rhenium and that technique generally results in losses of up to about 50 percent of the rhenium present due to volatilization. Additionally, after the recovery of rhenium by the extraction processes mentioned, the raffinate discarded contains 0.01 grams Re/liter. This concentration is often higher than that found in some concentrates. Also, reuse of the organic solution as the salt from perchlorate, thiocyanate, etc. which result after stripping with the agents previously disclosed, sharply lowers the extraction coefficients. The decrease in the extraction capacity with use is believed to be associated with the insolubility of the loaded amine in the decylalcoholamine-kerosene system.
It is believed, therefore, that a process that enables recovery of essentially all of the rhenium and an organic extraction solution that can be regenerated without an appreciable loss in extractive capacity is an advancement in the art.